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New hybrids proclaim their German engineering

New BMW cars that will be on display at the 2009 Frankfurt auto show reveal two possible futures for hybrid drivetrains in Europe and around the world.

I'm a big fan of "German engineering"--that combined focus on power and precision that distinguishes the better automobiles designed or manufactured in Germany.

At Frankfurt's International Motor Show, BMW will be showing off two new hybrid cars intended to deliver the full promise of German engineering. This is no small thing because most hybrid cars to date have been lightly built and somewhat underpowered in order to improve fuel economy.

The two new BMWs are more like previous offerings from that company: big, solid cars with lots of power to maintain performance in spite of the weight. They're also real cars, not just prototypes.

Fortunately, I don't need to describe these new cars here; there's a great article by Antuan Goodwin over on CNET's Car Tech blog that does a fine job of that (see "BMW unveiling two big hybrid models at Frankfurt"). What I would like to do instead is to drill down into their respective powertrains, which represent two different solutions to high-performance hybrid design, using images provided by BMW.

The ActiveHybrid X6, due to go on sale in the U.S. later this year, represents one end of the spectrum: higher-power electric motors and a larger battery pack. As the first image shows, the new X6 model has a twin-turbo V8 gas engine with 400 horsepower. Though this is a reasonably efficient engine for its size, it certainly wasn't chosen primarily for its fuel economy.

BMW ActiveHybrid X6 cutaway drawing
BMW's ActiveHybrid X6 uses a large NiMH battery pack and a new transmission with two integrated electric motors to augment its 400-horsepower gasoline engine. BMW

The large NiMH (nickel metal hydride) battery pack is also visible in this view, mounted above the rear drivetrain components and below the floor of the luggage compartment. This battery pack holds 2.4 kWh (kilowatt-hours) of energy; BMW also specifies a "nominal" 1.4 kWh figure, but doesn't explain the difference between total capacity and nominal capacity. I suspect the difference may be related to improving battery lifespan, or perhaps provide some extra storage to ensure that electric braking assist (regenerative braking) is always available. BMW does say that this braking mode can generate as much as 50 kilowatts of power and 0.3 g of deceleration force.

In the following view of the X6's transmission, the electric motors are shown built right into a new transmission that BMW describes as an "electric continuously variable transmission"--the electric motors and three planetary gearsets work together to provide the equivalent of seven gear ratios.

BMW ActiveHybrid X6 transmission cutaway drawing
The transmission in the ActiveHybrid X6 has two integrated electric motors totalling 177 horsepower. BMW

Interestingly, with ratings of 91 and 86 horsepower respectively, the two electric motors could provide a total of 177 horsepower, but the vehicle isn't designed to operate that way. BMW specifies a maximum system output for the ActiveHybrid X6 of 480 horsepower, reflecting a maximum contribution from the electric motors of around 80 horsepower.

Part of the issue here is that the battery pack is rated at a maximum output of 57 kW, roughly 76 horsepower, so there isn't enough electrical power to drive both motors at full power. I expect there are also some issues related to heat and torque, but BMW hasn't offered a full explanation of this limitation.

It's also interesting to note that while the new X6 supports a pure electric drive operating mode, it's only good up to 37 mph and 1.6 miles. This figure is well below the energy capacity of the battery pack, probably reflecting more limitations imposed in the name of long-term reliability.

The ActiveHybrid 7, a hybrid version of BMW's 7-series luxury sedan due out in the spring of 2010, takes a very different approach. It has only a small electric motor positioned between the engine and transmission, and it comes with only a small lithium-ion battery pack. The following figure shows that the battery pack is located to the side of the trunk, leaving plenty of space for golf bags. (BMW says that four standard bags will still fit, which I suppose is a critical specification for 7-series customers.)

BMW ActiveHybrid 7 cutaway drawing
The ActiveHybrid 7 is powered by a 440-horsepower gasoline engine and a small 20-horsepower electric motor with a lithium-ion battery pack. BMW

Zooming into the transmission in the following picture, we can see the pancake-style electric motor in front of what appears to be a fairly conventional 8-speed automatic transmission. In its press release, BMW does emphasize that this is a new transmission design "specifically tailored to the demands of hybrid technology," but the release doesn't explain how this gearbox differs from the 8-speed automatics on previous BMW cars.

BMW ActiveHybrid 7 transmission cutaway drawing
The ActiveHybrid 7's electric motor is tucked away between the gas engine and a conventional 8-speed automatic transmission. BMW

The new 7's electric motor produces a mere 15 kilowatts, roughly 20 horsepower. Together with the gas engine, the vehicle's maximum output is rated at 455 horsepower. The electric motor also functions as a starter motor for the gas engine and a generator to charge the two batteries on the car: a conventional 12V lead-acid battery and the 120V lithium-ion pack in the trunk.

The latter is a small pack storing only 400 watt-hours of energy--that's about like eight average laptop batteries. BMW doesn't mention whether the new 7 can run solely on the electric motor, but I doubt it; 20 horsepower probably isn't enough for that. Certainly the range would be negligible given the low battery capacity.

Instead, BMW describes the value of the electric motor in terms of two uses: first, it's powerful enough to restart the gas engine almost instantly (in less than one rotation of the crankshaft) so the engine can be shut down at stoplights, just as the Toyota Prius does. Second, the electric motor provides supplemental power while the gas engine is running, thus improving overall fuel efficiency. Since the battery can be charged by recovering braking energy, the power from the electric motor is often free.

BMW says the hybrid systems in the ActiveHybrid 7 were developed jointly with Daimler, maker of Mercedes automobiles. This partnership also led to some Mercedes models, and indeed, Mercedes has also introduced hybrids with powertrains similar to that of the ActiveHybrid X6. (Mercedes is announcing a new S500 hybrid in Frankfurt but didn't provide such nice pictures, so I didn't include it in this post.)

Both of these approaches will need to be developed substantially before they can reduce the total cost of owning and operating a motor vehicle. But it's clear that BMW, having waited this long to get into the hybrid car business, is giving its customers two very different choices. What the company does in the future will probably depend on how its customers respond.